1. Field of the Invention
The present invention relates to a high frequency module used in a communications apparatus for a wireless local area network (LAN), for example.
2. Description of the Related Art
Attention has been recently drawn to a wireless LAN that forms a LAN through the use of radio waves as a technique for constructing a network easily. A plurality of standards are provided for the wireless LAN, such as the IEEE 802.11b and the IEEE 802.11g that use a 2.4 GHz band as a frequency band and the IEEE 802.11a that uses a 5 GHz band as a frequency band. It is therefore required that communications apparatuses used for the wireless LAN conform to a plurality of standards.
Furthermore, the communications status on the wireless LAN varies depending on the location of the communications apparatus and the environment. It is therefore desirable to adopt a diversity for choosing one of a plurality of antennas whose communications status is best.
In the communications apparatus for the wireless LAN, a circuit portion that is connected to antennas and processes high frequency signals (the portion is hereinafter called a high frequency circuit section) is incorporated in a card-shaped adapter, for example. In addition, it is expected that the communications apparatus for the wireless LAN be installed in a mobile communications device such as a cellular phone. A reduction in size of the high frequency circuit section is therefore desired.
A type of mobile communications device such as a cellular phone is known, wherein a high frequency circuit section is formed as a module operable in a plurality of frequency bands. For example, Japanese Published Patent Application 2003-152588 discloses a module incorporating two diplexers and a single switch circuit. In this module the switch circuit switches one of the two diplexers to be connected to a single antenna. Each of the diplexers separates two signals in different frequency bands from each other.
Japanese Published Patent Application 10-145270 discloses a high frequency switch provided for a diversity that enables a transmission port and a reception port to be each switched to be connected to one of two antennas.
Japanese Published Patent Application 2002-64400 discloses a high frequency switch module operable in two frequency bands. This high frequency switch module comprises: a first switch circuit for switching transmission signals and reception signals of a first transmission/reception system; a first low-pass filter circuit connected to a transmission path of the first switch circuit; a second switch circuit for switching transmission signals and reception signals of a second transmission/reception system; a second low-pass filter circuit connected to a transmission path of the second switch circuit; a separator circuit for separating the first and second transmission/reception systems; and a layered structure for integrating the foregoing circuits.
Japanese Published Patent Application 2002-64400 discloses that transmission terminals and reception terminals are located in separate regions from each other with respect to a center line of the layered structure and that the arrangement of the transmission terminals and the reception terminals is line-symmetric. In addition, this publication discloses that, when antenna terminals, the transmission terminals and the reception terminals are called high frequency terminals, ground terminals are respectively disposed between adjacent ones of the high frequency terminals.
As described above, it is desirable that the communications apparatus for the wireless LAN conform to a plurality of standards whose operable frequency bands are different. It is therefore desired that the high frequency circuit section of the communications apparatus for the wireless LAN be capable of processing transmission signals and reception signals in a plurality of frequency bands. In addition, it is preferred that the communications apparatus for the wireless LAN adopt a diversity. Because of this, the high frequency circuit section of the communications apparatus for the wireless LAN preferably has a function of switching a plurality of antennas to be connected to an output port of reception signals and an input port of transmission signals. Furthermore, a reduction in size of the high frequency circuit section of the communications apparatus for the wireless LAN is desired.
In the module disclosed in Japanese Published Patent Application 2003-152588, a transmission circuit for processing transmission signals and a reception circuit for processing reception signals are placed inside a layered structure. As a result, there sometimes occurs electromagnetic coupling between the transmission circuit and the reception circuit in such a module. If such coupling is established, there occurs a problem that transmission signals leak from the transmission circuit to the reception circuit and/or reception signals leak from the reception circuit to the transmission circuit, and the isolation between the transmission circuit and the reception circuit is thereby degraded. This problem becomes more noticeable as the module is reduced in size. The problem therefore interferes with a reduction in size of the module.
In the module disclosed in Japanese Published Patent Application 2003-152588, no specific consideration is given to the arrangement of the terminals connected to the antennas and the terminals for receiving and outputting signals. In this case, it is difficult to separate the transmission circuit and the reception circuit from each other and it is therefore difficult to improve the isolation between the transmission circuit and the reception circuit. In this case, furthermore, it is required to separately design the transmission circuit and the reception circuit to be placed inside the layered structure, and it thus takes a longer period of time to design the transmission circuit and the reception circuit. Moreover, there tends to be unwanted portions of transmission lines provided in the layered structure, and it is therefore likely that loss and noise occurring in the module increase.
As previously described, Japanese Published Patent Application 2002-64400 discloses that the transmission terminals and the reception terminals are arranged in line-symmetry. However, this publication does not mention any specific consideration for the positional relationship between the transmission and reception circuits and the transmission and reception terminals. It is therefore difficult to solve the foregoing problems even if the technique disclosed in this publication is used.